The present invention relates to an optical transmitter for producing an optical signal indicative of the temperature of a component, such as a spark plug, operating under high tension. Such an optical transmitter is known (DE-C-2512634). The thermoelectric voltage from a hot junction on the tip of the insulator of the spark plug is applied to a voltage controlled oscillator (VCO) whose output frequency is thus dependent on and preferably proportional to the temperature to be measured. The oscillator output is fed to a light-emitting diode (LED) and the resulting pulsed optical signal is transmitted along an optical conductor to a light sensitive semi-conductor component where it is re-converted to an electrical signal. The electrical signal can be processed for further use, such as for the digital or analog display of the temperature or for appropriate control of the operation of the internal combustion engine to which the spark plug is fitted. The optical conductor electrically isolates the optical transmitter and thereby the thermoelectric element and its lead wires from the processing circuitry. Thereby the risk of arcing between the centre electrode of the spark plug and the thermo-electric element and its lead wires is minimized. By arranging the optical transmitter as close to the spark plug as possible, the radiation of interference and the capacitive loading on the ignition system are minimized. However, the sparks produced at the spark plug themselves have a very high frequency component (&lt;10.sup.9 Hz) and this can interfere with the evaluation of the measured voltage.